The present invention relates to lithographic printing plates or more particularly to a composition suitable for correcting the surface a metal based lithographic printing plate which has become scratched due to handling.
Lithographic is a printing system taking advantage of the natural mutual repulsion between water and oils. The surface of a lithographic printing plate comprises areas which accept water and repel an oily ink, and areas which repel water and accept an oily ink. The former areas constitute non-image areas, and the latter areas constitute image areas. Accordingly, if the surface properties between these two areas is disturbed, for example, if the hydrophilic property of the non-image areas is scratched during handling, inks are apt to adhere to such hydrophilicity-deteriorated areas to cause background stains.
Furthermore, when the non-image areas take scratches, the scratches are filled with an ink and are gradually rendered oilsensitive to cause stains.
One method of hydrophilically sensitizing a metal surface, particularly an aluminum surface, is by treating the surface with an aqueous alkali metal silicate solution such as a sodium or potassium meta-silicate solution. Such solutions have also been used to clean smudges, and to remove scratches and faint imperfections such as surface scratches from imaged and developed paper printing plates. Other alkaline salts of a strong base and a weak acid in addition to alkali metal silicates have been used in such solutions (See U.S. Pat. No. 3,394,653). U.S. Pat. No. 4,028,281 teaches a metal plate treating solution including an alkali metal silicate, a surfactant and the ferric chelate of ethylene diamine tetra acetic acid as a cleaning and scratch removing solution for imaged and developed planographic printing plates.
It has now been found that scratches in the background, non-image areas of plates may be corrected, i.e., re-hydrophilized so as not to accept greasy ink in the printing process, with a composition containing trisodium phosphate, sodium metasilicate, an anionic surfactant and water.